Electrochemical CO₂ reduction (CO₂RR) in zero-gap membrane electrode assemblies (MEAs) is a promising technology for industrial-scale CO production; however, its practical viability is hindered by carbonate formation and salt precipitation in neutral or alkaline environments. These phenomena deplete CO₂ reactants and obstruct active sites, limiting operational durability. While acidic media can circumvent these issues, the high proton activity typically promotes the competing hydrogen evolution reaction (HER) over CO₂RR.

In this work, we demonstrate a high-performance catalyst consisting of Ni₃S₂ nanostructures anchored on nitrogen-doped mesoporous carbon (Ni₃S₂@NMC-AL) specifically engineered for selective and stable CO conversion in acidic environments. The catalyst was synthesized via a nanocasting approach using SBA-15 as a hard template, followed by a critical acid leaching (AL) step. This purification process selectively removes unstable metallic and amorphous Ni species while preserving the sulfur-stabilized Ni₃S₂ active sites embedded within the robust, N-doped graphitic framework. Comprehensive structural characterization, including XRD, TEM-EDS, and XPS, confirmed that the Ni–S motifs are well-confined and remain stable even after exposure to harsh acidic conditions.

Performance was evaluated in a zero-gap MEA configuration under industrially relevant conditions. The optimized Ni₃S₂@NMC-AL catalyst exhibited CO Faradaic efficiencies (FE) exceeding 90% at current densities of 100 and 150 mA cm⁻². Notably, we found that CO selectivity remained largely independent of electrolyte buffer capacity (comparing KHCO₃ to K₂SO₄), indicating that performance is primarily governed by the local reaction environment and proton availability rather than bulk electrolyte identity.

By systematically tuning the anolyte pH, we identified moderate acidity (pH ≈ 3) as the optimal operating point, providing an ideal balance between suppressing HER and preventing salt deposition. Under these conditions, the catalyst achieved near-unity CO selectivity (98–99%) and exceptional long-term stability reaching 450 hours at 100 mA cm⁻². In contrast, while strongly acidic conditions (pH ≈ 1) also yielded high selectivity, they resulted in reduced durability due to accelerated HER kinetics and potential metal leaching.

These results highlight the synergistic role of Ni₃S₂ active sites, which weaken hydrogen adsorption and stabilize CO₂RR intermediates, in combination with the controlled proton transport provided by the MEA architecture. This study establishes a robust framework for designing acid-stable Ni-based catalysts and provides practical guidelines for optimizing local interfacial microenvironments in scalable CO₂ electrolysis systems.

Keywords: CO₂ reduction, Ni₃S₂ nanostructures, N-doped mesoporous carbon, Acidic CO₂RR, Zero-gap MEA, Long-term stability.